1. Field of the Invention
The present invention relates to optical devices for use in the field of optical communications.
2. Description of the Related Art
The use of optical fiber in long-distance transmission of voice and/or data is now common. As the demand for data carrying capacity in the transmission of voice and/or data continues to increase, there is a continuing need to augment the amount of actual fiber-optic cable being used as well as to utilize the bandwidth of existing fiber-optic cable more efficiently. One of the ways in which this last task may be performed is through the practice of wavelength division multiplexing (WDM) in which multiple information channels are independently transmitted over the same fiber using multiple wavelengths of light. In this practice, each light-wave-propagated information channel corresponds to light within a specific wavelength range or “band”. To increase data carrying capacity in a given direction, the number of such channels or bands should be preferably increased.
Additionally, it is desirable to use existing fiber for bidirectional communications. Through the use of WDM, a single optical fiber may be used to transmit, both simultaneously and independently, eastbound (northbound) as well as westbound (southbound) data. However, since all of the channels preferably reside within specific wavelength regions, determined by the properties of existing optical fiber or of other devices in the transmission system, such as optical amplifiers, increased channel capacity requires increased channel density. Thus, as the need for increased data carrying capacity escalates, the demand on WDM optical components—to transmit increasing numbers of more closely spaced channels with no interference or “crosstalk” between them and over long distances—becomes more severe.
For example, in a first typical channel allocation scheme, westward propagating channels may have a center wavelength comprised in a first, relatively short (“blue”), wavelength band and eastward propagating channels may have a center wavelength comprised in a second, relatively long (“red”), wavelength band. The “blue” wavelength band and the “red” wavelength band occupy separate wavelength ranges wholly contained in the optical transmission window centered near a wavelength of about 1.55 μm.
In a second typical channel allocation scheme, westward and eastward propagating channels may respectively have a center wavelength spaced by a predetermined channel spacing “d”. However, the center wavelengths of the eastward propagating channels are between the center wavelengths of the westward propagating channels (interleaved channels). For example, “even” channels λ2, λ4, λ6, λ8 may be westward propagating and “odd” channels λ1, λ3, λ5, λ7 may be eastward propagating.
Clearly, other channel allocation schemes may be used for implementing bidirectional optical communications. For the purposes of the present invention, it may be convenient to refer also to the optical frequency of the optical signals, in place of the wavelength thereof.
Back reflections of optical communications signals are a significant problem in optical systems. Such reflections may be generated at junctions between optical system components and/or may be due to scattering occurring along an optical fiber. They typically induce noise and distortion, which can significantly reduce and deteriorate the performance of a component and/or of the overall system. In particular, the back reflections are an acute problem in systems which include a gain element, such as an optical amplifier (either a rare earth doped amplifier or a semiconductor amplifier). In fact, reflections which travel back into the amplifier may be amplified and increase the error rate of the system or can cause the amplifier to randomly oscillate or begin to lase.
Optical isolators have been employed to inhibit reflections. To prevent oscillations or gain fluctuations occurring in the amplifier, isolators are usually employed at least at one end of an amplifier. Isolators are configured to allow optical signals to pass in one direction, but stop or inhibit signals traveling in the opposite direction.
In view of the difficulties caused by back reflections and the need to inhibit them with unidirectional isolators, gain elements are restricted to operating on signals transmitted in one direction. This imposes an increased cost burden on a system when gain is required in both directions of transmission on an optical fiber line, as in bidirectional optical communications.
However, devices suitable for allowing passage of optical signals of one wavelength band in one direction of travel and of optical signals of another wavelength band in the opposite direction of travel (and blocking back reflections in both cases) have already been proposed.
For example, U.S. Pat. No. 5,912,766, to Telstra Corporation Limited, discloses an optical isolator comprising two polarizer means, two input/output ports formed respectively on said polarizer means, and optical rotator means disposed between said polarizer means, said optical rotator means including Faraday rotator means and being selectively configured so that the isolator performs one of a plurality of isolator functions. The wavelength dispersion characteristics of said optical rotator means may determine said one of said isolator functions for at least two wavelength bands. In a disclosed embodiment, the isolator includes first and second input ports formed at the junction of respective graded-index (GRIN) lenses and spatial walk-off polarizers (SWPs). The isolator also includes a Faraday rotator and a reciprocal optical rotator disposed between the SWPs, such that all of the components form an in-line series assembly. The Faraday rotator and the reciprocal optical rotator are configured so as to provide one of a plurality of isolator functions for the isolator for two or more wavelength bands. For example, if λ1 and λ2 denote first and second wavelength bands, the functions may comprise isolate signals of λ2 in one direction and isolate signals of λ1 in the opposite direction, so that the isolator is allowed to function as a bidirectional isolator. The length of the Faraday rotator, which governs the length of the light transmission path therethrough, is selected so as to provide the rotator with a wavelength dispersion characteristic which gives rise to the desired polarization component rotation ±m 180°, where m is a non-negative integer. Similarly, the optical path length of the reciprocal optical rotator is selected to provide a wavelength dispersion characteristic which achieves the desired effective rotation ±m 180°. The reciprocal optical rotator may comprise half-wave plate or optically active material.
EP patent application No. 1,079,249, to JDS Uniphase Inc., discloses a bidirectional wavelength dependent optical isolator having two thick birefringent waveplates, having their optical axes oriented such that their birefringent axes are oriented differently, and a non-reciprocal element. The thick plates have a periodic wavelength response with polarization. In operation, even channels are passed while odd channels are blocked in a first direction from port 1 to port 2 and conversely, even channels are blocked and odd channels are passed in a second opposite direction from port 2 to port 1. In a disclosed embodiment, the first thick plate is half the length of the second thick plate and is oriented at 45° to vertically polarized incoming light and the second thick plate is oriented at 105° to the vertically polarized incoming light.
WO patent application No. 01/35131, to Avanex Corporation, discloses a bidirectional polarization independent optical isolator simultaneously transmitting two separate signal rays in opposite forward directions and simultaneously suppressing backward transmission of each signal ray in its respective reverse direction. The separate signal rays may comprise either two wavelength bands completely separated by wavelength (band bidirectional isolator) or two sets of wavelengths, such that wavelengths of the two signal rays are interspersed in alternating fashion (interleaved bidirectional isolator). The bidirectional polarization independent isolator includes a birefringent polarization separation/combining element, a reciprocal optical rotation element, a lens, a reflective element, and a reciprocal optical rotation element. The reflective element comprises either a mirror/waveplate assembly or a non-linear interferometer. More particularly, the mirror/waveplate assembly is disclosed in connection with the band bidirectional isolator and the non-linear interferometer is disclosed in connection with the interleaved bidirectional isolator. Four fibers or optical ports are optically coupled to the isolator and may be configured such that either single-stage bi-directional isolation is accomplished for each of two fiber transmission lines or double stage bi-directional isolation is accomplished on a single fiber transmission line.
EP patent application No. 1,191,808 discloses wavelength interleaving cross-connects that pass a first optical signal including a first set of optical frequencies in a first direction and a second optical signal including a second set of optical frequencies in a second direction. In one embodiment, the first optical signal, when input to a first input/output (I/O) port, is routed from the first I/O port to a third I/O port. The first optical signal, when input to a fourth I/O port, is routed from the fourth port to a second I/O port. The second optical signal, when input to the second I/O port, is routed from the second I/O port to the third I/O port. The second optical signal, when input to the fourth I/O port, is routed from the fourth I/O port to the first I/O port. Thus, by coupling an optical device (e.g., amplifier, filter) between the third port and the fourth port, the optical device can be used for bidirectional communications, thereby reducing the number of devices required for a bidirectional optical network architecture. Wavelength interleaving cross-connects disclosed in EP 1,191,808 are described in terms of filtering and routing even and odd International Telecommunications Union (ITU) channels. In one embodiment, the wavelength interleaving cross-connect has multiple half wave plates and two birefringent elements. A first half wave plate, a first birefringent element, a second half wave plate, a second birefringent element, and a third half wave plate together operate as a filtering element to filter optical signals that pass therethrough. The first birefringent element has optical path length of L and the second birefringent element has an optical path length of 2 L. In another embodiment, the wavelength interleaving cross-connect is combined with a pair of bidirectional isolators forming a uni-directional cross-connect with double-stage spectral isolation for use with an optical device, e.g. an amplifier. In another embodiment, the bidirectional isolators are combined into a single unit including a non-reciprocal rotator and a birefringent assembly.
U.S. patent application No. 2002/0024730 discloses bidirectional circulators based on interleaver technology, e.g. birefringent crystal interleaver technology, that enables signals containing even number ITU channels to travel in one direction through the device, while signals containing odd number ITU channels travel in opposite direction. In one embodiment, a bidirectional circulator is combined with a conventional three-port circulator to provide a four port device, which has two bidirectional ports and two uni-directional ports. Accordingly, signals traveling in opposite directions through a system can be passed in the same directions through an optical assembly coupled between the uni-directional ports. The optical assembly can be any one or more of: an erbium doped fiber amplifier, a fiber Bragg grating in transmission, a dynamic gain equalizer in transmission, a configurable add-drop multiplexer in transmission, a network monitoring device in transmission, and an isolating device.
The use of unidirectional optical isolators have also been proposed for applications exploiting Raman amplification. U.S. Pat. No. 5,673,280, to Lucent Technologies, discloses a low noise optical fiber Raman amplifier comprising an upstream and a downstream length of silica-based amplifier fiber, of combined length being more than 200 m, typically more than 1 km, with an optical isolator disposed between the upstream and downstream lengths of amplifier fiber, such that the passage of backscattered signal radiation from the latter to the former is substantially blocked. According to the authors, the provision of a multistage Raman amplifier, with an interstage isolator between adjacent stages, is an effective technique for reducing double Raman back scattering. Further, according to the authors, a multistage Raman amplifier with interstage isolator also increases the threshold for Brillouin scattering.
The Applicant observes that also in a unidirectional multistage Raman amplifier there may be signals that propagate in two opposite directions in the same fiber, i.e. the signal radiation amplified in the Raman amplifier fiber lengths and a counter-propagating pump radiation (typically a continuous wave signal) causing Raman amplification. In order to exploit Raman amplification, the pump wavelengths should be shifted in a lower wavelength region with respect to the signal wavelengths (typically with a shift of about 100 nm in silica-germania-based optical fibers). Advantageous configurations of counter-propagating Raman amplifiers may amplify an optical signal by using a first pump wavelength range in a first Raman fiber length and a second, different, pump wavelength range in a second Raman fiber length, so that three different wavelength ranges may be used in the same device (one for the signal, two for the pump radiation), two of which propagating in opposite directions with each other (see, for example, U.S. patent application No. 2002/0044335). However, the use of an isolator between the two Raman fiber lengths, as suggested in the above mentioned U.S. Pat. No. 5,673,280, would block the passage of the counter-propagating pump radiation from the downstream Raman fiber length to the upstream Raman fiber length. In preferred embodiments disclosed in '280, counter-propagating pump radiation is coupled into the downstream length of amplifier fiber, and wavelength-selective couplers are provided for shunting the pump radiation around the optical isolator. According to the Applicant, the use of a shunt circuit for allowing the counter-propagating pump radiation to propagate in the direction inhibited by the optical isolator may not represent an optimal solution, as it necessitates at least two more components, i.e. the wavelength selective couplers, increasing costs, complexity of the device and attenuation on the signal. The isolation requirements of the wavelength-selective couplers, which should be high in order to guarantee that signal and/or pump radiation are not lost in the shunt circuit, may be a further source of increasing costs.
The Applicant has tackled the problem of realizing optical devices being capable of passing and isolating signal radiations having different wavelengths traveling in opposite directions, i.e., bidirectional isolating devices. In particular, the Applicant has considered that different applications may require different schemes for the arrangement of allowed propagation directions (and forbidden directions) versus wavelength: for example, the opposite propagating signals may belong to mutually exclusive wavelength ranges or may have interleaved wavelengths; furthermore, the mutually exclusive wavelength ranges may be symmetrical or asymmetrical (i.e. they may have the same width or not); as another example, three or more different wavelength ranges may be used for the signal and the pump wavelengths in advantageous configurations of counter-propagating Raman amplifier. According to the Applicant, in such a complex framework the components included in a bidirectional isolating device should guarantee that, given a specific scheme, they may be simply reconfigured in order to allow the isolating device to comply with the specific scheme, without changing the type of device or the type of components included therein. In other words, the components included in the bidirectional isolating device should guarantee a high versatility of the device, in order to allow simple reconfigurations during the design thereof, according to the different requirements. Furthermore, the Applicant observes that another important feature that a bidirectional isolating device should comply with is the capability of allowing the use of as much wavelengths as possible for the opposite propagating signals, in order to maximally exploit the bandwidth available with optical fibers, in particular for WDM and dense WDM transmission: thus, the wavelength range dedicated to the transition between the wavelengths of optical signals allowed to travel in one direction and the wavelengths of optical signals allowed to travel in the opposite direction should be as small as possible.